1. Field of the Invention
The invention relates to a reflective liquid crystal display having a ferroelectric, chiral smectic liquid crystal layer.
The invention also relates to a projection system and a direct view display which has a liquid crystal display of the above-mentioned type.
The display according to the invention has, for example, a birefringent liquid crystal layer which is also referred to below as a Sc* layer and whose helical configuration is deformable by the action of an electric field.
2. Description of the Prior Art
A known liquid crystal display based on the deformation of a helical liquid crystal configuration is disclosed in EP-B-0 309 774. For example, the display disclosed in this prior publication and also referred to as a DHF cell (DHF represents Deformed Helix Ferroelectric) has a pair of transparent plates which together enclose the Sc* layer and which are each provided with a surface structure orienting a molecule of the Sc* layer, with electrodes for generating an electric field in the liquid crystal and with a polarizer.
The liquid crystals which can be used for DHF cells are distinguished by the fact that the molecules which belong to a smectic layer and are arranged essentially parallel to one another are arranged not perpendicular to the smectic plane but at a smectic tilt angle xcex8 to the normal of the plane. The chirality of the Sc* layer additionally results in the axes of the liquid crystal molecules being rotated relative to one another from layer to layer, so that consequently a screw-like helix having a pitch p forms.
In the rest state, i.e. without an applied electric field, the DHF cell exhibits a specific light transmittance. If a voltage is applied to the electrodes, an electric field is generated in the Sc* layer and reorients the molecules and thus causes a reorientation of the individual smectic layers. Application of the voltage thus deforms the helix of the Sc* layer, resulting in a change in the light transmittance.
A DHF display is based on a liquid crystal configuration which has a short pitch p. The pitch p is chosen so that the light is averaged over the coils of the helix, i.e. experiences only an average refractive index. For an undistorted helix, a birefringent layer having an optical axis parallel to the helix axis is thus obtained. If a voltage is applied between the electrodes of the two plates, in particular torques act on the individual smectic layers. These torques produce the stated deformation of the helix and consequently also a rotation of the optical axis in a plane parallel to the plates bounding the liquid crystal layer. The rotation of the optical axis as a function of the applied voltage can be determined in practice by means of the switching angle xcex1(U). The maximum switching angle a (Uxe2x89xa7Us) is a characteristic feature of the DHF cell and corresponds to the angle of rotation of the optical axis between the saturation voltage U=|Us| and U=0.
Reflective liquid crystal displays of the DHF type can be produced essentially by two different methods.
The first simpler method of production consists in mounting an optical mirror or a diffuse reflector on one side of a transmitting display cell known per se and consisting of a pair of transparent plates which together enclose a liquid crystal layer and are each provided with a surface structure and a polarizer. During operation of this first embodiment of reflective cells, the light then passes twice through the transmitting display cell. Most liquid crystal displays in clocks and watches, pocket calculators, etc. are based on this principle. Because the light passes a total of four times through a polarizer, large light losses occur in these displays. Reflective displays of this type additionally exhibit troublesome parallax effects at high resolution.
A second embodiment of reflective liquid crystal displays is shown in FIG. 1. In this display, the incident light 1 is linearly polarized by a polarizer 2, passes through the liquid crystal layer 3, is then reflected by a mirror 4 and passes once again through the liquid crystal layer 3 and the preceding polarizer 2. In this case, electrode and actuating means which can modify the birefringence properties of the liquid crystal according to the desired image information are additionally provided between the liquid crystal layer 3 and the mirror 4.
A major disadvantage of the display shown in FIG. 1 consists in the fact that it should ideally have a switching angle of xc2x145xc2x0 in the so-called symmetric mode. This is not possible with the liquid crystal mixtures known to date without resulting, even at small switching angles, in hysteresis effects which increase the switching time to an impermissible extent. Furthermore, the required voltage is greater for large switching angles than for small ones, which is another disadvantage of this type of display.
If, on the other hand, reflective DHF cells of the above-mentioned type are operated in the so-called asymmetric mode, the above-mentioned disadvantage can be at least partially eliminated with the asymmetric actuation. In this case, however, the electrical actuation cannot be effected by periodic pole reversal. This method of operation thus has a DC voltage component, which results in disadvantageous formation of space charges in the liquid crystal and at the interfaces. In fact, these space charges change the electrooptical characteristics, with the result that serious image defects and ghost images may form.
It is the object of the invention to provide a reflective DHF display which can be actuated without DC voltage and has a maximum switching angle a which is smaller than xc2x145xc2x0.
This object is achieved, according to the invention,by a liquid crystal display having
a chiral smectic, birefringent liquid crystal layer, whose optical axis can be electrooptically rotated in the plane of the display,
a polarizer and
a pair of parallel plates which enclose the liquid crystal layer and each of which is provided with a surface structure orienting the molecules of the liquid crystal layer and at least one electrode for generating an electric field, wherein
a light-reflecting mirror or a diffuse reflector being coordinated with one of the two plates,
a further birefringent layer or a further birefringent plate or film is arranged between the two plates,
the rotation of the optical axis of the liquid crystal layer is not more than xc2x122.5xc2x0 and
the display is formed in such a way that it can represent all gray values without DC voltage.
The use of the present invention is not restricted only to DHF cells but relates to all liquid crystal displays in which the electrooptical effect causes a rotation of the optical axis. It is particularly advantageous when the rotation of the optical axis during operation of the display shown in FIG. 1 is too small. Another class of liquid crystal cells whichxe2x80x94like the DHF cellxe2x80x94can be used for providing reflective displays of the type according to the invention is described in Swiss Patent Application No. 3073/96. Such cells are referred to below as APD (APD represents Alternating Polarization Domain) cells. Their advantages are low operating voltages and low currents. Like DHF cells, the APD cells too, in the embodiment according to the invention, have a maximum switching angle xcex1 which is smaller than xc2x145xc2x0.
The liquid crystal display according to the invention is distinguished in particular by the fact that the components of the reflective display described above with reference to FIG. 1 are also provided with a further birefringent element, in the form of an additional birefringent layer (or plate) or in the form of a second, switchable liquid crystal layer.
In a first category of liquid crystal displays according to the invention, this additional birefringent element is a solid, i.e. non-switchable plate 5 having an optical path difference xcex94nxc2x7d≈xcex/4 (where An denotes the birefringence and d the wall thickness of the plate 5 ), i.e. a quarter-wave plate whichxe2x80x94as shown in FIG. 2xe2x80x94is arranged after the liquid crystal layer 3 in the direction of incidence of the light, and in particular between said layer and the mirror 4. Since the light passes twice through the plate 5 during operation of the display, the optical path differences of said plate sum to xcex/2, for which reason the plate 5 can also be designated, in terms of a model, as a half-wave plate or half-wave layer.
In a second category of liquid crystal displays according to the invention, the additional birefringent component is a second switchable liquid crystal layer which is arranged in the form of a compensator 105, described in more detail below, between the polarizer 102 and the specified liquid crystal layer 103. FIG. 3 shows a corresponding configuration, from which it is evident that in this case the incident light 101 is first linearly polarized by the polarizer 102 then passes through the upcircuit compensator 105 and the liquid crystal layer 103, is reflected by the mirror 104 and then passes once again through the liquid crystal layer 103, the cell 105 and the preceding polarizer 102.